Oriented blends of polypropylene and polybutene-1

ABSTRACT

THIS INVENTION IS DIRECTED TO ORIENTING AND HEAT SEALING AND HEAT SHRINKING BLENDS OF ISOSTATIC POLYPROPYLENE AND POLYBUTENE-1.

April 30, 1974 H. G. SCHIRMER 3,393,304

ORIENTED BLENDS 0F POLYPROPYLENE AND POLYBU'I'ENE-l Original Filed March18, 1964 2 Sheets-Sheet 1 SEAL STRENGTH OF POLYPROPYLENE POLYBUTENE-lBLENDS LEGEND fmg'yfl ORIENTED IOO/ O loo /5O Fig.1

RACKING TEMPERATURE RANGES o m E LIJ v (D 80 2 8 g POLYPROPYLENE/POLYBUTENE-l Fig. 2.

April 30, 1974 H. G. SCHIRMER ,8 3,

ORIENTED BLENDS OF POLYPROPYLENE AND POLYBUTENE-l Original Filed March18, 1964 2 Sheets-Sheet 2 SHRINK PROPERTIES OF POLYPROPYLENE/POLYBUTENE-l 'BLENDS HAT VOLUME (CC) FBLEND(WT.%POLYPROPYLENE/WWI.POLYBUTENE-I) IOO 50/50 I o/loo Fig. 3

CRYSTALLINE MELT POINT iBLENDSMT.%POLYPROPYLENE/Wt% POLYBUTENE I i Fig.4

United States Patent Int. Cl. C08f 29/12 US. Cl. 264--289 8 ClaimsABSTRACT OF THE DISCLOSURE This invention is directed to orienting andheat sealing and heat shrinking blends of isotactic polypropylene andpolybutene-l.

This application is a continuation of application Ser. No. 355,522,filed Mar. 18, 1964, now abandoned.

This invention relates to oriented blends of polypropylene andpolybutene-l. In one aspect the invention relates to a method forimproving the heat scalability of polypropylene. In another aspect theinvention relates to a method for improving the heat shrinkability oforiented polypropylene.

Polypropylene and polybutene-l are known thermoplastic mono-l-olefinpolymers. More recently, new copolymers of alphaolefins (of the formulaCH =CHR Where R is a hydrocarbon radical) have been discovered which arelinear, regular head-to-tail, have substantially no branches longer thanR, and comprise macromolecules having a regular steric structure whichhas been referred to as an isotactic structure. The isotactic structureis the structure of the portions of the macromolecules in which, if themacromolecule is arbitrarily assumed to be fully extended in a plane,the R substituents on the tertiary carbon atoms are all on one side(e.g. above) and the H atoms on those carbon atoms are all on the otherside (e.g. below) of the plane of the chain. (In a successive isotacticportion of the same chain the position of the R and H substituents maybe reversed with respect to the position they had in the previousposition.) The isotactic macromolecules are defined herein as thosemacromolecules which are non-extractable with boiling n-heptane forpolypropylene and diethylether for polybutene-l, are highlycrystallizable, and are crystalline under appropriate conditions. Forpurpose of this discussion the isotactic content is determined byplacing 2.5 to 0.1 gram of polymer in a weighed extraction thim'ble andextracting in an ASTM Rubber Extraction Apparatus for 2.5 hours with 100ml. of boiling normal heptane (for polypropylene or diethylether forpolybutene-l) at one atmospheric pressure. The thimble is then removedand dried in a forced oven at 110 C. for 2 hours after which it iscooled in a desiccator and weighed. The weight percent of residue, basedon original polymer, is calculated and recorded as isotactic content. Asemployed herein, atactic content refers to the extractable portion ofthe polymer and the term crystalline is essentially synonymous withisotactic and amorphous refers to atactic polymer.

Isotactic polypropylene has a high ultimate strength, a high elasticmodulus and retains useful mechanical properties at higher temperaturesthan most thermoplastic resinous materials. It tends to be, however,brittle at about 20 C. and is diflicult to process. Further, its use inapplications where resistance at low temperatures is required tends tobe limited. In addition it will not heat seal to itself at temperaturesless than its melting point (about 340 P.) which requires sealingtemperatures so high as to restrict its use for oriented film.

Isotactic polybutene-l has a high tensile strength and a relatively lowbrittle point (about 4' F.) but has a low elastic modulus which is alimit in applications requiring flexural rigidity.

It is an object of the invention to provide a novel blend ofpolypropylene and polybutene-l.

Another object of the invention is to provide a method of improving theseal strength of polypropylene.

Yet another object is to improve the seal strength of a blend ofpolypropylene and polybutene-l.

Still another object is to provide a heat shrinkable polypropylene.

Another object is to provide a heat shrinkable blend of polypropyleneand polybutene-l.

Still another object is to provide a method of heat shrinking orientedpolypropylene.

Another object is to provide a method of heat shrinking an orientedblend of polypropylene and polybutene-l.

These and other objects of the invention will be readily apparent tothose skilled in the art from the following disclosure and appendedclaims.

These objects are broadly accomplished by preparing a blend ofpolypropylene and polybutene-l and heat sealin g said blend at atemperature less than the melting point of either polymer. In oneembodiment of the invention, the blend of polymers is oriented by coldworking and heat shrunk at low temperatures.

Although blends of polypropylene and polybutene-l are known, it is mostsurprising to discover that these blends, particularly blends containingabout 30-50 weight percent polypropylene, have unusual and usefulproperties. Most advantageous to the film users is the discovery thatthese blends may be heat sealed at a temperature lower than thecrystalline melt point of either polymer. High seal strengths areobtained at much lower temperatures than one would expect from the knownproperties of either polymer. Further, the polymer blend may be orientedor non-oriented. The oriented polymer blend has the additional advantageof being heat shrinkable at much lower temperatures than one wouldexpect from the known properties of either polymer.

The blend will have variable properties depending on the ratio ofpolypropylene to polybutene-l. Desirable properties are obtained whenthe blend contains 30-90, more preferably 30-50, weight parts ofpolypropylene and 70-10, more preferably 70-50, weight parts ofpolybutene-l, based on total blend weight.

The orientation of thermoplastic polymers, such as polyethylene,polypropylene, polybutene-l and copolymers of vinylidene chloride, bycold working is Well known. The invention is not limited to anyparticular method of orientation and any suitable means, such asstretched either unilaterally or bilaterally, may be employed. Forsimplification, orientation is described herein with reference tobiaxial orientation (actually omniaxial) wherein molten polymer(including a blend) is extruded through an annular die to form a tubewhich is solidified. The tube, or tape, is then inflated by internal airpressure at a temperature less than its crystalline melting pointthereby biaxially orienting the molecules. The resultant bubble is thendeflated to form a flattened tube of tape which may be slit to form afilm.

The racking temperatures and internal air pressures are fairly criticalfor each type of polymer. If the temperature is too high or too low thebubble will break thus stopping production until a new bubble can beformed. These conditions have been found to be especially critical forpolypropylene or polybutene-l, particularly isotactic polypropylene orisotactic polybutene-l. However, it has now been found that the additionof polybutene-l to polypropylene, or vice versa, greatly expands thepermissible temperature limits for racking of either of these polymers.

Further the required temperatue is lowered as the concentration ofpolybutene-l increases. Strangely, at a constant work load (constantexpansion volume) the permissible racking temperature is reduced about910 F. for every 10% polybutene-l added to polypropylene until about 60%has been added where the temperature drop suddently increases to about17 F. (less amounts for less work) for each 10%. This abrupt drop inracking temperature in the area of 60% polybutene-l indicates a decidedchange in the orientation mechanism, although the exact mechanism canonly be guessed. In addition, an important advantage of the blend is theclarity of the racked film.

It has also been found that a blend of polypropylene and polybutene-l isheat sealable at a temperature less than the melting point of eitherpolymer by itself. Sealing at a temperature in excess of the meltingpoint, under pressure, produces almost complete fusion, but results inloss in strength of the surrounding film area due to the change fromcrystalline to amorphous structure. However, polypropylene has beenfound to be only slightly sealable at temperatures below its melt pointand, although polybutene-l melts at a lower temperature, about 251 F.,it lacks the strength of polypropylene. It was therefore most surprisingto discover that polypropylene polybutene-l blends could be heat sealedat temperatures less than 220 F.-even as low as 160 -F. Neither polymerby itself can be heat sealed at these temperatures. The strength of theseal depends on the temperature employed and the blend ratio as depictedin FIG. 1. The strength of the seal increases with the increase inpolybutene-l concentration up to about 60% polybutene-l. Atconcentrations greater than 60% polybutene-I, the seal strengthdiminishes. Seal strengths were not measured above the melting point ofthe respective blends. Seal strengths for both the oriented andnon-oriented blends are shown. These are further described in ExampleII. Preferably the sealing temperature is less than 290 F., morepreferably 160 to 250 F., even more preferably 160 to 210 F. The sealsof oriented polymer thus produced have less deformation due to shrinkthan seals produced at higher temperatures.

An inherent disadvantage of oriented polypropylene is the hightemperature required to shrink the film. Surprisingly, the addition ofpolybutene-l lowers the shrink temperature so that substantial shrinkenergies may be obtained at temperatures of 212 F. or even less. Thismakes it possible to use boiling Water to shrink the film instead ofsome fluid at much elevated temperatures to achieve shrinkage. Again,peculiar properties of blends of about -50% polypropylene are observed.The amount of shrinkage at a particular temperature increases as theconcentration of polybutene-l increases with a significant increase atabout polypropylene. As the concentration of polybutene-l furtherincreases there is a sharp decrease in shrinkage until the concentrationreaches about 80% polybutene-l when the amount of shrinkage levels outor increases with the increase of polybutene-l concentration. Attemperatures less than about 240 F. the amount of shrinkage for the40/60 polypropyene/polybutene-l blend is actually greater than foreither of the polymers by themselves. The temperature employed dependson the polymer blend but is preferably 160 to 290 F., more preferably190 to 230 F., even more preferably about the boiling point of water.

The polymers employed to prepare the oriented homogeneous blend of theinvention are polypropylene and polybutene-l, more preferably anisotactic polypropylene and an isotactic polybutene-l. Preferably, theisotactic content of both the polybutene-l and the polypropylene is atleast 93%, preferably about 97 to 100%, as measured by the polymersinsolubility in boiling n-heptane for polypropylene or boilingdiethyletber for polybutene-l as hereinbefore described.

The polymers may be admixed by any suitable means to form a homogeneousblend, such as dry mixing, solution mixing, or mixing the two polymerstogether while in a molten state or combinations thereof.

during the blending operation.

No irradiation or crosslinkage of the films is required.

ples.

EXAMPLE I Biaxial orientation of blends of polypropylene andpolybutene-l A series of 15 mil tapes were chill cast from homogeneousmixtures of isotactic polypropylene (6420, Hercules Powder Co., 98.2%insoluble in boiling n-heptane) and isotactic polybutene-l (Petro-TexAS8-121, 97.2% insoluble in boiling diethyletber. The barrel temperatureof the extruder was 375 F. and the die temperature was 420 F. The chillroll temperature was 225 F. except that the roll temperatures waslowered to 160 F., F., 130 F., and 125 F. for the 70, 80, 90, and 100%polybutene-l runs, respectively. Starting with 100% by weightpolypropylene, mixtures were cast with the amount of polybutene-lincreasing at 10% intervals until 100% polybutene-l was reached.

The tapes of the polymer blends were then biaxially oriented by blowinga hatf This was accomplished by placing a platen of the tape over aconduit in communication with air under 7 p.s.i. pressure. The edges ofthe platen were held in place and the entire platen heated to aspecified temperature. When the platen reached this temperature, asdetermined by thermocouples, a valve was automatically opened to permitthe air pressure to blow a bubble or hat in the platen. This isconsidered to be generally equivalent to racking wherein a tape isbiaxial- 1y oriented by inflating the tape between 2 sets of rolls toform a trapped bubble as known to those skilled in the art. If the hatfails at the employed air pressure, the temperature employed is eithertoo high or too low. The range of racking temperatures for each blendwas determined by racking each tape at 7 p.s.i. air pressure, startingat 290 F. and progressively lowering the temperature at 10 F. incrementsuntil the tape no longer racked into film. A plot showing the maximumracking temperatures vs. the blend ratios is shown in FIG. 2.

It will be seen from FIG. 2 that as polybutene-l concentration wasincreased, the racking temperature range was greatly expanded. When ablend containing at least 70% polybutene-l is oriented the racking rangeis surprisingly extended greatly until 100% polybutene-l where themaximum racking temperature abruptly drops to 190 F. It is obvious fromFIG. 4 that these sealing temperatures are unexpected from thecrystalline melt point' determinations of the blends which give noevidence of a eutectic point. These melt points were made by observingthe loss of birefringance patterns produced by polarized light withinthe polymer sample as the sample was heated. Complete loss of thebirefringance pattern was interpreted to be the crystalline meltingpoint of the. polymer.

EXAMPLE II Seal strength of blends of polypropylene and polybutene-lvThe seal strength ofvthe heat seals of films oriented as described inExample I and sealed at F. to 280 F.

Seal strength was determined with a constant rate of loading on atensile tester. The linear dimensions ofthe The invention is bestdescribed by the following examsealed surface were held constant at 1inch. One piece of the sample was held in one clamp while the otherpiece was held in the other. As the rate of loading was increased, thenet effect was to peel the seal apart. The force required to do this wasmeasured in grams and the strength of the seal was reported as the gramsforce/ linear inch of seal required to completely peel the seal apart.

The relative seal strength of each blend is depicted in FIG. 1.Surprisingly, the film blends heat seal to each other even at sealtemperatures less than the melting point of the lowest melting polymer,viz. polybutene-l, which melts at about 251 F. Although seal strengthincreases with temperature, there was considerable strength in the 40/60 blends even at temperatures as low as 160 F. The breadth, as well asthe strength, of the blend ratios which seal increased at 180 F. At 200F. seal strength greatly increased. No attempt was made to determineseal strength of the blends at a temperature above the melting point ofthe blends since complete fusion would result and the seal strengthwould approach the strength of the unoriented film.

The seal strength of non-oriented blends of polypropylene/polybutene-lwas determined by reextruding the aforementioned blends and chillcasting the extrudate. Seal strength was determined as hereinbeforedescribed and plotted in FIG. 1. It is to be noted that again the sealstrength was sufficient to be of practical value even at temperatures aslow as 160 F. even though non-oriented polymer does not have thestrength of oriented polymer.

A major advantage of the ability of the blend to heat seal attemperatures less than the melting point of either polymer is that thereis less effect on the alignment of the molecules so that the inherentstrength of the oriented film can be utilized.

Since neither polypropylene nor polybutene-l seals be low 220 F., onewould expect that the blends also would not seal at these temperatures.However, as can be seen in FIG. 1, most blends could be satisfactorilysealed below this temperature. The strength of the seals increased withthe increase of polybutene-l concentration up to about 60% polybutene-l.At concentrations greater than about 60% polybutene-l the seal strengthdiminished sharply. The low temperature heat scalability of the blendsis thus seen to be a peculiar characteristic of the blends and notsimply a proportional additive effect of the polybutene-l.

EXAMPLE III Free shrinkage of blends of polypropylene and polybutene-lUsing the same hat samples oriented in Example I one hour afterextrusion, the free shrinkage was obtained by subjecting each of thesamples to gradually increasing temperatures and measuring the loss oforiginal volume at the reference temperature (70 F). Each sample wascoated with oil to avoid heat sealing and was maintained at the testingtemperature for five minutes. Shrink temperature was then plotted vs.hat volume (cc.) (not shown). Since all of the heat seal data wereobtained from hats oriented to 800 cc. volume, interpolated shrinkvalues were obtained from the above (not shown) plot for each blend andplotted in FIG. 3. The constant temperature lines in FIG. 3 reveal ahigher tendency for shrinkage as the amounts of polybutene-l wereincreased up to about the 40/ 60 blend. Surprisingly, as theconcentration of polybutene-l increased above 60%, the degree of filmshrinkage at any one temperature sharply decreased and then eitherremained constant or increased again depending on the shrinktemperature. Thus, it will be seen that considerable shrinkage (about25% at 212 F. for the 40/60 blend) is obtained at temperaturesconsiderably less than those normally employed for heat shrinking ofeither of the parent polymers. Thus, boiling water, for example, couldbe used.

6 EXAMPLE 1v Overlap seal with oriented blends of polypropylene andpolybutene-l Using the same polymers as in Example I, 300 pounds of ablend of isotactic polypropylene and 20% isotactic polybutene-l weremixed in pellet form. A 2%- inch extruder equipped with a six inchtubular die was started using polypropylene. The blend was thensubstituted and the die and barrel temperatures maintained at 350 F.Both clarity and thickness remained constant throughout the entirefive-hour run.

The following day two rolls of 6 inch tubing were racked at various tapespeeds of 8 to 12 feet per minute at about 300 F. by inflating the tapewith air to form a bubble between two sets of pinch rolls. Thelongitudinal racking ratio was about 7.0 and the transverse rackingratio varied from 4:9 (31 /2 inch film) to 5.9 (38 inch film) dependingon the amount of air. The bubble was then deflated and the tape slit toform a .75 mil film.

Samples of the film were then run on automatic breadwrap equipment.Effective overlap seals were made over a wide range of temperatures,220270 F., with the best seal being made at 240 F. No end labels wererequired. No difiiculty in sealing was encountered even though manylayers of film.

EXAMPLE V Two hundred pounds of polybutene-l (isotactic content-97.5%)were mixed with 180 pounds of polypropylene (Profax 6420) to produce a40/ 60 blend. This was extruded onto a 6 inch tape at 350 F., quenchedwith water and biaxially oriented at 180 F. by inflating into a .75 milfilm in a 2 day run using a 2% inch oven extruder. Rapid quenching (2%inches from die) produced an extremely clear film exhibiting up to 40%shrink at 205 F. By lowering the cooling ring 2 feet below the die theresulting tapes were crystalline enough to withstand a 200 F. rackingtemperature without welding and yet yielding clear films. Film wassuccessfully oriented from 180 F. to 230 F. but optimum conditionsexisted where the tape was oriented at 210 F.

EXAMPLE VI TABLE I Seal strength Degree of quenching gms./linear inchRapid 144 Medium 78 Slow 52 Rapid quench refers to a water quench 2%"below the die. Medium quench refers to a water quench 2 feet below thedie. Slow quench refers to a water quench 6 feet below the die. Since arapid quench should produce a polymer which is more amorphous than onewhich has been slowly quenched, it is apparent that the heat seals ofthe more amorphous tapes had higher strength than the films producedfrom more crystalline tapes.

Another advantage of the blend of the invention is that film madetherefrom may be simultaneously heat sealed and heat shrunk atconvenient temperatures such as the boiling point of water. For example,an object to be enclosed is wrapped in a film of the blend, the filmedges overlapped, and the wrapped object submerged in boiling waterthereby not only shrinking the film tightly around the object but alsosealing the film together at the points of contact.

While certain examples, structures, composition and process steps havebeen described for purposes of illustration, the invention is notlimited to these. Variation and modification within the scope of thedisclosure and the claims can readily be effected by those skilled inthe art.

I claim:

1. A process for heat sealing and heat shrinking isotactic polyproylenecomprising blending 30/90 weight parts of isotactic polypropylene with10-70 weight parts of polybutene-l to form a homogeneous blend, forminga film by extruding said blend, water quenching said film at not morethan 2 feet from the extrusion die, biaxially orienting said film at atemperature between 180 F.230 F juxtapositioning a portion of said filmwith a surface, applying heat to said film at a temperature below themelting point of said polypropylene and below the melting point of saidpolybutene-l and between 160 to 250 F. to seal said film and applyingheat between 180 to 290 F. to shrink said film.

2. The process of claim 1 wherein said film is water quenched at notmore than 2 /8 inches from the extrusion die.

3. The process of claim 1 wherein said orientation is carried out atabout 210 F.

4. A process for producing an isotactic polypropylene material havingsuperior heat sealing and heat shrinking characteristics comprisingblending 3090 weight parts of isotactic polypropylene with 10-70 weightparts of polybutene-l to form a homogeneous blend, forming a film byextruding said blend, orienting said film at a temperature between 180F. and 230 F.

5. The process of claim 4 wherein said blend is 30-50 weight parts ofisotactic polypropylene and 70-50 weight parts of isotactic polybutene-land wherein said extruded film is water quenched at not more than 2 feetfrom the extrusion die and orientation is carried out biaxially at about210 F.

6. A process for heat sealing isotactic polypropylene comprisingblending 30-90 weight parts of isotactic polypropylene with 10-70 weightparts of polybutene-1 to form a homogeneous blend, forming a film fromsaid blend, biaxially orienting said film, juxtapositioning a portion ofsaid film with a surface, and sub-merging said film in boiling water tosimultaneously heat seal and heat shrink same, said water temperaturebeing about 212 F.

7. A process for heat sealing isotactic polypropylene comprising:blending 30 to weight parts of isotactic polypropylene which is at least93% insoluble in boiling normal heptane with from to 50 weight parts ofisotactic polybutene-l which is at least 93% insoluble in boilingdiethylether to form a homogeneous blend; forming a film from saidblend; biaxially orienting said film; juxtapositioning a portion of saidfilm with a surface; heat sealing the film at a temperature in the rangeof F. to 210 F.; and, heat shrinking the film at least 25% at atemperature in the range of F. to 230 F.

8. The process of claim 6 wherein an object is at least partiallyenclosed in said film, portions of said film are overlapped, and saidfilm is thereafter placed in boiling water and simultaneously heatsealed and heat shrunk.

References Cited UNITED STATES PATENTS 3,372,049 3/1968 Schaifausen 11773,246,061 4/1966 Blatz 26495 3,014,234 12/1961 Koppehele 18l 3,022,5432/1962 Baird et al 1857 FOREIGN PATENTS 809,484 2/ 1959 Great Britain260897 MURRAY TILLMAN, Primary Examiner C. J. SECCURO, AssistantExaminer US. Cl. X.R.

